Siltation in the Saltmarsh of the Dee Estuary Derived from 137Cs Analysis of Shallow Cores

Shallow cores, 50-100 cm long, extracted at 28 locations in the saltmarsh of the Dee Estuary were analysed for 137Cs content at 2-cm intervals. These depth profiles of 137Cs were correlated against a 40-year record of 137Cs discharges from the nuclear fuel reprocessing plant at Sellafield, some 130 km to the north. To allow for a non-uniform rate of deposition in the saltmarsh, a series of non-linear depth-time relationships was assumed for each core. The best fit of 137Cs core profiles and 137 Cs discharge records then provided estimates of the most likely rate of deposition at each location. The resulting deposition rates are spatially coherent, lending confidence to these estimates. Over the last 20 years, net deposition ranged from 5 cm at the head of the estuary to 60 cm at the offshore edge closest to the sea. These deposition rates have decreased steadily over this 40-year period with net deposition over the last 20 years only half that of the first 20 years. Net deposition throughout the entire saltmarsh over the last 20 years is 6 × 106 m3, far more than the available supply of sediment from cliff erosion. This amount of deposition represents approximately 2•8% of the net volume of sediment transported by tides in and out of the adjacent Mersey Estuary, an estuary of similar dimensions, over this period. The total amount of 137Cs trapped in the saltmarsh represents approximately 1/3000 of the total emitted from Sellafield. The depth-time relationships derived from this 137Cs study permit corresponding studies of the magnetic properties in nine of these cores. Results showed a coherent pattern of dispersion away from a source at the head of the estuary. Both the location and depositional history inferred from these data are compatible with the operational history of a major steelworks.